This invention relates generally to imaging systems capable of operation in multiple modalities, and more particularly to an apparatus and method for aligning a multi-modality imaging system.
Multi-modal imaging systems are capable of scanning using different modalities, such as, for example, but not limited to, Positron Emission Tomography (PET) and Computed Tomography (CT). The difference between multi-mode and multi-modality is that in multi-mode systems the same hardware is utilized to perform scans in different modes (e.g., a radiation source and a radiation detector is used in both a flouro mode and a tomosynthesis mode), while in a multi-modal system (multi-modality system), although some of the same hardware is utilized to perform different scans (e.g., an image produced by PET is processed and displayed respectively, by the same computer and display as an image produced by CT), the data acquisition systems (hereinafter sometimes termed “modality unit”) are different. For example, on a CT/PET system a radiation source and a radiation detector are used in tandem to acquire CT data, while a radiopharmaceutical is typically employed in tandem with a PET camera to acquire PET data. It is contemplated that the benefits of the invention accrue to all multi-modality imaging systems, such as, for example, but not limited to, a CT/PET imaging system as well as systems utilizing currently unknown modalities as well as currently unfeasible combinations, such as, for example, but not limited to, a combination PET/ultrasound system and/or a CT/MRI system.
In multi-modality systems, for example, an integrated PET-CT system the PET and CT should be inherently registered with one another. Since the patient lies still on the same table during the PET and CT portions of the acquisition, the patient should be in a consistent position and orientation during the two acquisitions, greatly simplifying the process of correlating and fusing the CT and PET images. The CT image is then used to provide attenuation correction information for the reconstruction of PET images. An image reader correlates the anatomic information presented in the CT image and the functional information presented in the PET image. Inherent registration of the CT and PET images assumes a perfect alignment of the PET and CT detector coordinate systems, or at least a known spatial transformation between the two coordinate systems. However, misalignment of the coordinate systems will directly result in a mis-registration of the images.
One previously proposed method for aligning components of a multi-modality image system is to compare the positions of a known attenuation object using both CT and PET attenuation measurements. For example, to align a CT imaging system fabricated as part of the multimodality imaging system the attenuation object is irradiated with an x-ray beam. An electrical signal that represents the intensity of the impinging x-ray beam and attenuation of the x-ray beam is received at a detector. When the second imaging modality is a PET imaging system the PET imaging system is modified to generate attenuation data. Specifically, additional equipment is installed on the PET system to obtain the attenuation data. While accurate, this process may be time consuming and limited by the statistical uncertainty in the transmission acquisition. As a result, utilizing attenuation data to align both the CT imaging system and the PET imaging system results in increased costs of the overall dual imaging system and an increased time to perform the alignment.